1. Field of the Invention
This invention relates to a camera apparatus such as a video camera which produces an image signal and more particularly to a camera apparatus which produces an image signal from which a fixed pattern noise has been removed.
2. Description of the Related Art
It is considered important for a video camera, particularly a high-sensitivity camera of the kind to be used under a low degree of illumination, to remove from the image signal an intrinsic noise of the image sensing part of the camera (hereinafter referred to as a fixed pattern noise) as much as possible. An apparatus developed to meet this requirement has been disclosed in an article entitled "High-Sensitivity Camera and Image Processing Technique" which appeared in "NHK Technical Research Monthly Report," Oct., 1986, Vol. 29, p 375-378. The arrangement of this known apparatus is as described below with reference to FIG. 1 of the accompanying drawings:
Referring to FIG. 1, the light of an object to be photographed comes through an optical system 10 to fall on the imaging plane of an image sensing part 12 and is photo-electric converted there. The photo-electric conversion output of the image sensing part 12 is supplied to a camera signal processing circuit 14. The circuit 14 performs a known signal processing operation to convert it into a known image signal form of NTSC,
or the like. The output of the camera signal processing circuit 14 is supplied to one of the input terminals of a subtracter 16 and also to an image memory 20 via a normally open switch 18. The switch 18 operates in association with a shield plate 22 disposed in front of the optical system 10. While the incidence of the light of a photographing field on the optical system 10 is blocked by the shield plate 22, the switch 14 is closed to allow the output of the camera signal processing circuit 14 to be stored in the image memory 20. This allows the image memory 20 to store a fixed pattern noise including the intrinsic noises of the image sensing part 12 and the ensuing circuit system. This fixed pattern noise is taken into the image memory 20 either at the start of a photographing operation or during a pause thereof.
The image memory 20 is arranged to output the stored signal in synchronism with the image signal outputting action of the camera signal processing circuit 14. A multiplier 24 is arranged to multiply the output of the image memory 20 by a suitable constant k. The output of the multiplier 24 is supplied to the other input terminal of the subtracter 16. The subtracter 16 subtracts the output (a fixed pattern noise component) of the multiplier 24 from the output of the camera signal processing circuit 14. This arrangement gives an image signal with the fixed pattern noise reduced to a sufficiently low level.
In the case of a solid-state image sensor, the fixed pattern noise is considered to result mainly from the following causes: (i) uneven sensitivity due to uneven light receiving areas of picture elements; (ii) a thermal noise dependent upon the ambient temperature; and (iii) uneven amplification rates of amplifiers in a case where different amplifiers are provided for different picture elements. While the noise resulting from the causes (i) and (ii) remains unchanged from its initial state, the thermal noise (iii) changes any time during the process of a photographing operation.
The thermal noise greatly affects visual sensation. Therefore, with a fixed pattern noise initially taken in by the image memory 20 at the beginning of photographing, if the above-stated subtracting operation is carried on by using the initial noise value over a long period of time, the noise component of the output image signal would gradually increase.
To avoid this, the noise value stored in the image memory 20 must be renewed taking the fixed pattern noise again into the memory 20 by giving pause to the photographing operation before completion thereof. The renewal work is not only troublesome but also makes a long photographing operation impossible. Besides, the method of doing the renewal work is hardly a fundamental solution of the problem because the noise component of the output image signal gradually increases before the renewal work.